An antenna of the above description is disclosed in the Applicant's co-pending British Patent Application which has been published under the number 2292638A, the subject matter of which is incorporated in this specification by reference. In its preferred form, the antenna of that application has a cylindrical ceramic core, the volume of the solid ceramic material of the core occupying at least 50% of the internal volume of the envelope defined by the elongate conductive elements and the sleeve, with the elements lying on an outer cylindrical surface of the core.
The antenna is particularly intended for the reception of circularly polarised signals from sources which may be directly above the antenna, i.e on its axis, or at a location a few degrees above a plane perpendicular to the antenna axis and passing through the antenna, or from sources located anywhere in the solid angle between these extremes. Such signals include the signals transmitted by satellites of a satellite navigation system such as GPS (Global Positioning System). To receive such signals, the elongate conductive elements comprise four coextensive helical elements having a common central axis which is the axis of the core, the elements being arranged as two laterally opposed pairs of elements, with the elements of one pair having a longer electrical length than the elements of the other pair. Such an antenna has advantages over air-cored antennas of robustness and small size, and over patch antennas of relatively uniform gain over the solid angle within which transmitting satellite sources are positioned.